Anthropod-borne viral diseases remain very significant causes of morbidity and mortality in humans. Many arboviruses in enzootic cycles through a phenomenon known as Transovarial Transmission (TOT). TOT refers to the transmission of the arbovirus from an infected female to her progeny and thus allows for maintenance of the virus outside of the vertebrate host. TOT is critical in temperate regions because it allows trans-seasonal maintenance of the arbovirus through the winter in its diapausing invertebrate host. The Crosse Encephalitis (LAC) virus (Bunyaviridae) is the most important cause of pediatric viral encephalitis in the United States. TOT of LAC virus by its vector, the woodland treehole mosquito Aedes triseriatus, is one of the best characterized TOT systems and has become the model system for studying TOT biology. However, virtually nothing is known about the genetic basis for TOT in Ae. triseriatus or LAC. The principal goals of this proposal are to quantify the genetic basis of TOT of LAC virus in Ae. triseriatus and map the vector genes involved. Marker-assisted selection (MAS) will then be used to select vector strains that are homozygous at all TO-relevant loci. MAS TOT refractory strains will be used to identify anatomical barriers to TOT while MAS TOT susceptible strains will be used to assess the role the genetic variation in different LAC isolates plays in TOT.